Recent advances in data processing technology have spurred the development of a number of high speed devices for rendering permanent records of information. Alphanumeric non-impact printing mechanisms now include thermal, electrostatic, magnetic, electrophotographic, and ionic systems. Of particular import in these developing systems has been ink jet printing technology, because it offers a simple and direct method of electronically controlling the printed output and has the special advantage of being non-contact, high speed, and particularly well adapted to plain paper printing.
Generally, ink jet systems can be categorized into three basic types: continuous droplet ink jets in which droplets are generated continuously at a constant rate under constant ink pressure, electrostatically generated ink jets, and ink-on-demand jets (or impulse jets). This invention is concerned primarily with this latter system.
The primary approach in commercially available ink-on-demand systems has been to use piezoelectric crystals to propel ink from the orifice of a tube of narrow cross-section. A typical example of this approach is described in U.S. Pat. No. 3,832,579 entitled PULSED DROPLET EJECTING SYSTEM issued Aug. 27, 1974, by J. P. Arndt. Here a small cylindrical piezoelectric transducer is tightly bound to the outer surface of a cylindrical nozzle. Ink is brought to the nozzle by an ink hose connected between the broad end of the nozzle and an ink reservoir. As the transducer receives an electrical impulse, it generates a pressure wave which accelerates ink toward both ends of the nozzle. An ink droplet is formed when the ink pressure wave exceeds the surface tension of the meniscus at the orifice on the small end of the nozzle.
In these piezoelectric ink jet systems, a principal problem is associated with the relative disparity in size between the piezoelectric transducer and the ink jet orifice. The transducer is generally substantially larger than the orifice, thereby limiting either the minimum separation of the jets or the number of jets which can be used on a given print head. Furthermore, piezoelectric transducers are relatively expensive to produce and are not amenable to many of the modern semiconductor fabrication techniques.
Another type of ink-on-demand system is described in U.S. Pat. No. 3,174,042 entitled SUDDEN STEAM PRINTER issued June 28, 1962 by M. Naiman. This system utilizes plurality of ink containing tubes. Electrodes in the tubes contact the ink and upon a trigger signal an electric current is passed through the ink itself. This current flow heats the ink by virtue of a high I.sup.2 R loss (where I is the current and R is the resistance of the ink), vaporizes a portion of the ink in the tubes, and causes ink and ink vapor to be expelled from the tubes.
The principal drawbacks of this steam-type system are the serious difficulties in controlling the ink spray, and the constraints on ink conductivity, since a highly conducting ink requires a large current flow to achieve the required vaporization, and therefore unduly restricts the types of ink which might be used.
Despite the fact that both of these systems have been known for many years, the technology of ink-on-demand ink-jet printing has yet to resolve the fundamental problems associated with each of these devices.